last-modified: fix use of uninitialized memory

git-last-modified(1) uses a scratch bitmap to keep track of paths that have been changed between commits. To avoid reallocating a bitmap on each call of process_parent(), the scratch bitmap is kept and reused. Although, it seems an incorrect length is passed to memset(3). `struct bitmap` uses `eword_t` to for internal storage. This type is typedef'd to uint64_t. To fully zero the memory used by the bitmap, multiply the length (saved in `struct bitmap::word_alloc`) by the size of `eword_t`. Reported-by: Anders Kaseorg <andersk@mit.edu> Helped-by: Jeff King <peff@peff.net> Signed-off-by: Toon Claes <toon@iotcl.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>